在Android中使用RC4进行图像加密

Question

I have implemented the rc4 algorithm myself, works like a charm in encrypting and decrypting texts, the problem comes when I try to encrypt/decrypt an image. My output image is always unreadable. No exceptions are thrown. I'm not sure whether its the encryption or decryption that doesn't work, or maybe both. Your help will be appreciated.

RC4 Class

public class RC4 {

private byte[] S = new byte[256];

public RC4 (byte[] key) {

     byte[] T = new byte[256];

    int keylen, j;
    byte t;

    for (int i = 0; i < 256; i++) {

        keylen = key.length;
        S[i] = (byte) i;
        T[i] = key[i % keylen];
    }

    //KSA -  key-scheduling algorithm is used to initialize the permutation in the array "S"
    j = 0;
    for (int i = 0 ; i < 256 ; i++)
    {
        j = ((j + S[i] + T[i]) % 256) & 0xFF;

        //swap
        t = S[i];
        S[i] = S[j];
        S[j] = t;
    }
}

public byte[] encrypt(byte[] plaintext)
{
    int j = 0, i = 0, t, k;
    byte temp;
    byte[] pt,ct, s;

    //deep copy
    s = S.clone();

    pt = plaintext;
    ct = new byte[pt.length];
    for (int jj = 0 ; jj < pt.length; jj++)
    {
        i = ((i + 1) % 256) & 0xFF;
        j = ((j + s[i]) % 256) & 0xFF;

        //classic swap
        temp    = s[jj];
        s[jj]   = s[j];
        s[j]    = temp;

        t = ((s[i] + s[j]) % 256) & 0xFF;

        k = s[t];

        ct[jj] = (byte) (k ^ pt[jj]);
    }
    return ct;
}

public byte[] decrypt(byte[] ciphertext)
{
    return encrypt(ciphertext);
}

}

Then my calls: Server Side

Bitmap SelectedImage = BitmapFactory.decodeFile(selectedImagePath);
            byte[] b = bitmapToByteArray(SelectedImage);

            RC4 rc4 = new RC4(sharedSecret);
            byte[] encrypted = rc4.encrypt(b);
            ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
            objectOutputStream.writeObject(encrypted);
            objectOutputStream.flush();

Client Side

FileOutputStream fos = null;
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
                byte[] b = (byte[])objectInputStream.readObject();

                RC4 rc4 = new RC4(sharedSecret);
                byte[] decrypted = rc4.decrypt(b);

                Bitmap bmp = BitmapFactory.decodeByteArray(decrypted, 0, decrypted.length);
                fos = new FileOutputStream(file);
                bmp.compress(Bitmap.CompressFormat.PNG, 100, fos);
                fos.close();

Answer 1

I discovered my error, it was in the encrypt function; misuse of the counter. Following is the corrected code.

for (int counter = 0 ; counter < pt.length; counter++)
    {
        i = ((i + 1) % 256) & 0xFF;
        j = ((j + s[i]) % 256) & 0xFF;

        //classic swap
        temp    = s[j];
        s[j]    = s[i];
        s[i]    = temp;

        t = ((s[i] + s[j]) % 256) & 0xFF;

        k = s[t];

        ct[counter] = (byte) (k ^ pt[counter]);